1. Field of Invention
The present invention relates to a voltage regulator. More particularly, the present invention relates to a voltage regulator capable of improving system response. The operational amplifier inside the voltage regulator is able to maintain a fast response without having to reduce the frequency bandwidth of the amplifier.
2. Description of Related Art
Conventionally, a voltage regulator is constructed using an operational amplifier (OP). To have a fast response, open circuit gain of the operational amplifier must increase. However, increasing the gain of the amplifier must be accompanied by a decrease in operational bandwidth, otherwise system instability may occur.
FIG. 1 is a block diagram of a conventional voltage regulator. As shown in FIG. 1, the voltage regulator 10 consists of an operational amplifier 12 and a feedback circuit 14. The operational amplifier 12 further includes a pre-amplifier 16 and a power amplifier 18. The pre-amplifier 16 is a signal integrator and an amplifier, and the power amplifier 18 is a device for increasing the power of incoming signals. There is also a large loading between the pre-amplifier circuit 16 and the power amplifier circuit 18. The feedback circuit 14 is a signal-attenuating device. The pre-amplifier 16 has two input terminals. One input terminal of the pre-amplifier 16 is connected to the output terminal of the feedback circuit 14, while the other input terminal is connected to an input voltage terminal V.sub.IN. The input of the power amplifier 18 is coupled to the output terminal of the pre-amplifier 16. The output terminal of the power amplifier 18 is coupled to the input terminal of the feedback circuit 14 and an output voltage terminal V.sub.OUT. In addition, one terminal of a load element 20 is coupled to the output voltage terminal V.sub.OUT while the other end is coupled to the ground GND.
Signals from the output voltage terminal V.sub.OUT are returned to the input terminal of the pre-amplifier 16 via the feedback circuit 14. Due to attenuation by the loading on the load element 20 and the feedback circuit 14, only a small fraction of the original output signal at the output terminal V.sub.OUT is returned to the pre-amplifier 16. The pre-amplifier 16 inside the operational amplifier 12 amplifies the differential voltage between signal at the input terminal V.sub.IN and the feedback signal. The amplified signal is fed to the large loading between the pre-amplifier 16 and the power amplifier 18. Finally, signal power is boosted up by the power amplifier 18 and output to the load element 20.
Because the signal has to pass through various load elements connected to the voltage regulator 10, speed of response for the operational amplifier 12 is reduced. However, to maintain the speed of response by increasing the operational bandwidth of the operational amplifier 12, the gain of the pre-amplifier 16 must be reduced. However, a reduction of the pre-amplifier gain reduces the capability of the pre-amplifier 16 for driving the power amplifier 18, thereby affecting the response of the entire system.